As such a type of conventional inverter device, for example, Japanese Patent Unexamined Publication No. 2003-189670 discloses a device that works on a sine-wave drive system by PWM 2-phase modulation.
Hereinafter will be described the aforementioned drive system. FIG. 23 is an electric circuit diagram showing a sine-wave driving inverter device and the peripheral circuits thereof. Control circuit 107 of inverter device 121 detects the position of magnet rotor 105 that constitutes sensorless DC brushless motor 111 by calculating current values fed from current sensor 106. According to an rpm instruction signal (not shown) and the like, control circuit 107 controls switching elements 102 of inverter circuit 110 so that DC voltage from battery 101 is switched by PWM modulation. Through the modulation, sinusoidal wave-shaped AC current is fed to stator winding 104 of sensorless DC brushless motor (hereinafter, motor) 111.
Diodes 103 of inverter circuit 110 form a return route of current from stator winding 104. For the purpose of the explanations given hereinafter, it will be assumed that switching elements 102 are formed of upper-arm switching elements (2U, 2V, 2W) and lower-arm switching elements (2X, 2Y, 2Z), and switching elements 2U, 2V, 2W, 2X, 2Y and 2Z correspond to diodes 3U, 3V, 3W, 3X, 3Y and 3Z, respectively.
The current value detected by current sensor 106 is used for calculating power consumption and used as a judgment indicator of protecting switching elements 102 and the like. Although current sensor 106 is disposed on the minus side of the power supply line in FIG. 23, it may be disposed on the plus side, since both the sides carry a same amount of current.
FIG. 24 and FIG. 25 show characteristics of waveforms (i.e., U-phase terminal voltage 141, V-phase terminal voltage 142, W-phase terminal voltage 143 and neutral-point voltage 129) in two-phase modulation; FIG. 24 shows each waveform in two-phase modulation with a maximum modulation degree of 50%, and FIG. 25 shows the waveforms in the modulation with a maximum modulation degree of 100%. FIGS. 24 and 25 show that each terminal voltage is applied as pulse voltage with pulse width of duty (%) shown in the vertical axis of each graph. (For convenience in the description, the wording ‘duty’ represents the ratio of the ON period to the sum of the ON period and the OFF period.) Neutral-point voltage 129 is obtained by dividing the sum of the terminal voltage values by 3. The phase voltage exhibits sinusoidal wave, which is obtained by subtracting the value of the neutral-point voltage from the value of the terminal voltage.
FIG. 26 is a timing chart of two-phase modulation in one carrier (a carrier cycle), showing an on/off state of the upper-arm switching elements 2U, 2V, 2W and the lower-arm switching elements 2X, 2Y, 2Z. The timing chart corresponds to the phase of about 135° in two-phase modulation with a maximum modulation degree of 50% shown in FIG. 24. There are three switching patterns (a), (b) and (c), each of the current paths is shown in the electric circuit diagrams of FIG. 27A through FIG. 27C.
In the period of pattern (a), all of upper-arm switching elements 2U, 2V, 2W are turned off, and all of lower-arm switching elements 2X, 2Y, 2Z are turned on. The U-phase current and the V-phase current flow from the diodes parallel to lower-arm switching elements 2X and 2Y, respectively, to stator winding 104. The W-phase current flows from stator winding 104 to lower-arm switching element 2Z. The current flows between the lower-arm switching elements and motor 111. That is, current is not fed from battery 101 to inverter circuit 110 and motor 111.
In the period of pattern (b), upper-arm switching element 2U and lower-arm switching elements 2Y, 2Z are turned on. In this period, the U-phase current flows from upper-arm switching element 2U to stator winding 104; the V-phase current flows from the diode parallel to lower-arm switching element 2Y to stator winding 104; and the W-phase current flows from stator winding 104 to lower-arm switching element 2Z. That is, the current is fed from battery 101 to inverter circuit 110 and motor 111. In this period, the power supply line (current sensor 106) carries the U-phase current.
In the period of pattern (c), upper-arm switching elements 2U, 2V and lower-arm switching elements 2Z are turned on. In this period, the U-phase current and the V-phase current flow from upper-arm switching elements 2U and 2V, respectively, to stator winding 104; and the W-phase current flows from stator winding 104 to lower-arm switching element 2Z. That is, current is fed from battery 101 to inverter circuit 110 and motor 111. In this period, the power supply line (current sensor 106) carries the W-phase current.
The on/off state of upper-arm switching elements 2U, 2V, 2W tells that whether or not the power supply line (current sensor 106) carries current, and which phase of current flows when current is detected. That is, when all three phases are turned off, no current flows (non-conducting state); when only one phase is turned on, the current corresponding to the phase flows (conducting state); and when two phases are turned on, the current corresponding to the remaining phase flows (conducting state).
FIG. 28 shows the ON period of upper-arm switching elements 2U, 2V, 2W in one carrier (a carrier cycle) at phases of 90°, 105°, 120°, 135° and 150° in FIG. 24 (i.e., in the two-phase modulation with a maximum modulation degree of 50%). The ON period (duty) of the upper-arm switching elements is equally shown on the left and right sides from the middle of a carrier cycle. In the figure, a thin solid line represents the ON period of the U-phase; a medium solid line represents the V-phase; and a thick solid line represents the W-phase. In addition, under the ON period, the conducting period during which power supply is fed from battery 101 to stator winding 104 is indicated by an arrowed solid line, and the flowing phase current in the period is indicated by capital letters of U, V, W. The non-conducting period is indicated by an arrowed broken line. Similarly, FIG. 29 shows the ON period of the upper-arm switching elements at each phase in the two-phase modulation with a maximum modulation degree of 100% shown in FIG. 25.
In a carrier (carrier cycle) in the two-phase modulation, regardless of the phase current, the conducting period—in which electric power is fed to inverter circuit 110 and motor 111—appears once, even in a different phase.
Next will be described three-phase modulation. FIG. 30 and FIG. 31 show characteristics of waves in three-phase modulation with a maximum modulation degree of 50% and 100%, respectively. Like the two-phase modulation described above, FIGS. 30 and 31 show U-phase terminal voltage 141, V-phase terminal voltage 142, W-phase terminal voltage 143 and neutral-point voltage 129. FIG. 30 and FIG. 31 show that each terminal voltage is applied as pulse voltage with pulse width of duty (%) shown in the vertical axis of each graph. (For convenience in the description, the wording ‘duty’ represents the ratio of the ON period to the sum of the ON period and the OFF period.) Neutral-point voltage 129 is obtained by dividing the sum of the terminal voltage values by 3. The phase voltage exhibits sinusoidal wave, which is obtained by subtracting the value of the neutral-point voltage from the value of the terminal voltage.
FIG. 32 is a timing chart of three-phase modulation, showing an on/off state of the upper-arm switching elements (2U, 2V, 2W) and the lower-arm switching elements (2X, 2Y, 2Z) in one carrier (a carrier cycle). The timing chart corresponds to the phase of about 120° in the three-phase modulation with a maximum modulation degree of 50% shown in FIG. 30.
The switching pattern of the switching elements of the three-phase modulation has further period (d) in addition to periods (a), (b) and (c) described in the two-phase modulation. The periods (a), (b) and (c) in the three-phase modulation are the same as those in the two-phase modulation shown in FIGS. 27A through 27C and therefore the description here will be given on period (d).
In period (d), as shown in FIG. 33, all three upper-arm switching elements 2U, 2V, 2W are turned on, and all three lower-arm switching elements 2X, 2Y, 2Z are turned off. The U-phase current and V-phase current flow from upper-arm switching elements 2U and 2V, respectively, to stator winding 104. The W-phase current flows from stator winding 104 to the diode parallel to upper-arm switching element 2W. The current flows between the upper-arm switching elements and motor 111. That is, current is not fed from battery 101 to inverter circuit 110 and motor 111.
The on/off state of upper-arm switching elements 2U, 2V, 2W tells that whether or not the power supply line (current sensor 106) carries current, and which phase of current flows when current is detected. That is, when all three phases are turned off, no current flows (non-conducting state); when only one phase is turned on, the current corresponding to the phase flows (conducting state); when two phases are turned on, the current corresponding to the remaining phase flows (conducting state); and when three phases are all turned on, no current flows (non-conducting state).
FIG. 34 shows the ON period of upper-arm switching elements 2U, 2V, 2W in one carrier (a carrier cycle) at phases of 30°, 45°, 60°, 75° and 90° in FIG. 30 (i.e., in the three-phase modulation with a maximum modulation degree of 50%). The ON period (duty) of the upper-arm switching elements is equally shown on the left and right sides from the middle of a carrier cycle. In the figure, a thin solid line represents the ON period of the U-phase; a medium solid line represents the V-phase; and a thick solid line represents the W-phase. The conducting period during which power supply is fed from battery 101 to stator winding 104 is indicated by an arrowed solid line, and the flowing phase current in the period is indicated by capital letters of U, V, W. The non-conducting period is indicated by an arrowed broken line.
Similarly, FIG. 35 shows the ON period of the upper-arm switching elements at each phase in the three-phase modulation with a maximum modulation degree of 100% shown in FIG. 31. In the three-phase modulation, as shown in FIGS. 34 and 35, period (d) in the middle of a carrier cycle is a non-conducting period. The non-conducting period also appears in the beginning and the end of the carrier cycle. That is, a conducting period appears twice—one is in the first half of the carrier cycle; and the other is in the latter half of the cycle. Compared to the two-phase modulation where the conducting period appears once, the three-phase modulation has a carrier cycle shortened to half, i.e., the carrier frequency is double (hereinafter referred to as a carrier cycle-shortening effect), by which a fine and smooth PWM modulation is obtained. As compared to the two-phase modulation, the three-phase modulation generally exhibits less current ripple and torque ripple, thereby further reducing noise and vibration. However, there are some exceptions that can't offer the carrier cycle-shortening effect in the three-phase modulation above. In a carrier cycle at a phase of 30° in the modulation with a maximum modulation degree of 100%, as shown in FIG. 35, the conducting period appears once in the cycle, and therefore the carrier cycle-shortening effect cannot be obtained. Similarly, in a carrier cycle at a phase of 90° in FIG. 35, due to absence of the non-conducting period in the beginning and the end of a carrier cycle, the conducting period is linked with the ones in the previous carrier cycle and the successive cycle. Although a carrier cycle at a phase of 90° contains two conducting periods, it is equivalent to one conducting period per carrier cycle. As a result, the carrier cycle-shortening effect cannot be obtained.
In an inverter device that works on a sine-wave drive system by PWM modulation, in terms of reducing noise and vibration, the three-phase modulation is generally effective, than the two-phase modulation, in providing the carrier cycle-shortening effect. However, in the modulation with a maximum modulation degree of 100%, there are some cases where noise and vibration cannot be reduced to a desired level due to lack of the carrier cycle-shortening effect.